Cytokine pathways mediate a broad range of biological functions, including many aspects of inflammation and immunity. Janus kinases (JAK), including JAK1, JAK2, JAK3 and TYK2, are cytoplasmic protein kinases that associate with type I and type II cytokine receptors and regulate cytokine signal transduction. Cytokine engagement with cognate receptors triggers activation of receptor associated JAKs and this leads to JAK-mediated tyrosine phosphorylation of signal transducer and activator of transcription (STAT) proteins and ultimately transcriptional activation of specific gene sets (Schindler et al., 2007, J. Biol. Chem. 282: 20059-63). JAK1, JAK2 and TYK2 exhibit broad patterns of gene expression, while JAK3 expression is limited to leukocytes. Cytokine receptors are typically functional as heterodimers, and as a result, more than one type of JAK kinase is usually associated with cytokine receptor complexes. The specific JAKs associated with different cytokine receptor complexes have been determined in many cases through genetic studies and corroborated by other experimental evidence. Exemplary therapeutic benefits of the inhibition of JAK enzymes are discussed, for example, in International Application No. WO 2013/014567.
JAK1 was initially identified in a screen for novel kinases (Wilks A. F., 1989, Proc. Natl. Acad. Sci. U.S.A. 86:1603-1607). Genetic and biochemical studies have shown that JAK1 is functionally and physically associated with the type I interferon (e.g., IFNalpha), type II interferon (e.g., IFNgamma), and IL-2 and IL-6 cytokine receptor complexes (Kisseleva et al., 2002, Gene 285:1-24; Levy et al., 2005, Nat. Rev. Mol. Cell Biol. 3:651-662; O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). JAK1 knockout mice die perinatally due to defects in LIF receptor signalling (Kisseleva et al., 2002, Gene 285:1-24; O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). Characterization of tissues derived from JAK1 knockout mice demonstrated critical roles for this kinase in the IFN, IL-10, IL-2/IL-4 and IL-6 pathways. A humanized monoclonal antibody targeting the IL-6 pathway (Tocilizumab) has been approved by the European Commission for the treatment of moderate-to-severe rheumatoid arthritis (Scheinecker et al., 2009, Nat. Rev. Drug Discov. 8:273-274).
CD4 T cells play an important role in asthma pathogenesis through the production of TH2 cytokines within the lung, including IL-4, IL-9 and IL-13 (Cohn et al., 2004, Annu. Rev. Immunol. 22:789-815). IL-4 and IL-13 induce increased mucus production, recruitment of eosinophils to the lung, and increased production of IgE (Kasaian et al., 2008, Biochem. Pharmacol. 76(2): 147-155). IL-9 leads to mast cell activation, which exacerbates the asthma symptoms (Kearley et al., 2011, Am. J. Resp. Crit. Care Med., 183(7): 865-875). The IL-4Rα chain activates JAK1 and binds to either IL-4 or IL-13 when combined with the common gamma chain or the IL-13Rα1 chain respectively (Pernis et al., 2002, J. Clin. Invest. 109(10):1279-1283). The common gamma chain can also combine with IL-9Rα to bind to IL-9, and IL-9Rα activates JAK1 as well (Demoulin et al., 1996, Mol. Cell Biol. 16(9):4710-4716). While the common gamma chain activates JAK3, it has been shown that JAK1 is dominant over JAK3, and inhibition of JAK1 is sufficient to inactivate signaling through the common gamma chain despite JAK3 activity (Haan et al., 2011, Chem. Biol. 18(3):314-323). Inhibition of IL-4, IL-13 and IL-9 signaling by blocking the JAK/STAT signaling pathway can alleviate asthmatic symptoms in pre-clinical lung inflammation models (Mathew et al., 2001, J. Exp. Med. 193(9): 1087-1096; Kudlacz et. al., 2008, Eur. J. Pharmacol. 582(1-3): 154-161).
Biochemical and genetic studies have shown an association between JAK2 and single-chain (e.g., EPO), IL-3 and interferon gamma cytokine receptor families (Kisseleva et al., 2002, Gene 285:1-24; Levy et al., 2005, Nat. Rev. Mol. Cell Biol. 3:651-662; O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). Consistent with this, JAK2 knockout mice die of anemia (O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). Kinase activating mutations in JAK2 (e.g., JAK2 V617F) are associated with myeloproliferative disorders in humans.
JAK3 associates exclusively with the gamma common cytokine receptor chain, which is present in the IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 cytokine receptor complexes. JAK3 is critical for lymphoid cell development and proliferation and mutations in JAK3 result in severe combined immunodeficiency (SCID) (O'Shea et al., 2002, Cell, 109 (suppl.): S121-S131). Based on its role in regulating lymphocytes, JAK3 and JAK3-mediated pathways have been targeted for immunosuppressive indications (e.g., transplantation rejection and rheumatoid arthritis) (Baslund et al., 2005, Arthritis & Rheumatism 52:2686-2692; Changelian et al., 2003, Science 302: 875-878).
TYK2 associates with the type I interferon (e.g., IFNalpha), IL-6, IL-10, IL-12 and IL-23 cytokine receptor complexes (Kisseleva et al., 2002, Gene 285:1-24; Watford, W. T. & O'Shea, J. J., 2006, Immunity 25:695-697). Consistent with this, primary cells derived from a TYK2 deficient human are defective in type I interferon, IL-6, IL-10, IL-12 and IL-23 signalling. A fully human monoclonal antibody targeting the shared p40 subunit of the IL-12 and IL-23 cytokines (Ustekinumab) was recently approved by the European Commission for the treatment of moderate-to-severe plaque psoriasis (Krueger et al., 2007, N. Engl. J. Med. 356:580-92; Reich et al., 2009, Nat. Rev. Drug Discov. 8:355-356). In addition, an antibody targeting the IL-12 and IL-23 pathways underwent clinical trials for treating Crohn's Disease (Mannon et al., 2004, N. Engl. J. Med. 351:2069-79).
Currently there remains a need for additional compounds that are inhibitors of Janus kinases. For example, there is a need for compounds that possess useful potency as inhibitors of one or more Janus kinases (e.g., JAK1) in combination with other pharmacological properties that are necessary to achieve a useful therapeutic benefit. For example, there is a need for potent compounds that demonstrate selectivity for one Janus kinase over other kinases in general. There is also a need for potent compounds that demonstrate selectivity for one Janus kinase over other Janus kinases (e.g., selectivity for JAK1 over other Janus kinases). Kinases demonstrating selectivity for JAK1 could provide a therapeutic benefit, with fewer side effects, in conditions responsive to the inhibition of JAK1. Additionally there is currently a need for potent JAK1 inhibitors that possess other properties (e.g., melting point, pK, solubility, etc.) necessary for formulation and administration by inhalation. Such compounds would be particularly useful for treating conditions such as, for example, asthma.